Government and industry reeling after Russian hackers pierce internal networks

WASHINGTON — The discovery this weekend that Russian hackers used sophisticated techniques to infiltrate a broad swath of government and corporate networks to steal sensitive information sent cybersecurity experts into a panic, leaving open the question of how the U.S. will respond.

The attribution was made on Sunday, according to one source familiar with the matter, who said the spy group responsible for the breach is known to the military and intelligence community. U.S. Cyber Command, the military combatant command charged with pursuing U.S. enemies in cyberspace, is closely involved...